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cerveza_fiesta
06-19-2014, 02:20 PM
Plans for many older boats specify rot-resistant hardwoods like oak for structural members (keels, stems, sternpost, etc...).

If one were doing a conversion to fiberglass/cedar strip planking on, say, a 16' meloseed or a 12' sailing dinghy, what would be the best material to use in place of the oak?

Reason I ask is the known problem with epoxy and its poor adhesion to certain types of oak.

I know I have a lot more to worry about than this in such a conversion, it's just a question that's been nagging at me because I can't seem to find a clear answer anywhere. Do you just substitute in a less rot-resistant species (white ash for example) and count on the fiberglass/epoxy to keep the water out? Would it be better maybe to just beef up the board in question and use cedar instead?

Probably opening a can of opinion-worms here too...just wondering in general what people's thoughts are on the matter.

Thorne
06-19-2014, 02:41 PM
I know I have a lot more to worry about than this in such a conversion, it's just a question that's been nagging at me because I can't seem to find a clear answer anywhere. Do you just substitute in a less rot-resistant species (white ash for example) and count on the fiberglass/epoxy to keep the water out? Would it be better maybe to just beef up the board in question and use cedar instead?



No to the first, Yes to the second. Question is way too general to really answer. Much depends on how hard the boat will be sailed, as some designs might want white oak in certain frames or planks.

I think you mean white oak, and some of the newer epoxies like G-Flex and the Smith & Co.'s "Oily Wood" epoxies are generally regarded as working well.

Todd Bradshaw
06-19-2014, 06:37 PM
I've used white ash for this purpose on occasion. Mostly it's function was improved fastener crush resistance over the softwood in places where runners or keels would eventually be screwed to the outside of the hull (finish washers, bedding compound, epoxy-sealed holes, and screws run through the stripped hull). Rot resistance really doesn't play much of a part in the equation here. If you get a break where water is getting into a stripper's core, you have a serious problem that needs to be dried out and fixed immediately. By the time anything in there rots, you'll have a far worse problem going on with delamination of the fiberglass. It's pretty simple - a stripper's core must stay dry for the duration. Otherwise, the boat comes apart.

In situations where the piece is not getting a lot of concentrated localized stress (say a transom for example, which would have been solid hardwood on the traditionally built version) the easiest thing to do is probably to thicken up your strip core a bit with your "normal" cedar or whatever you're using and if needed, increase the glass weight a bit on that part (on both sides).

Top left photo - the strip running down the middle of the bottom is ash. There is an oak runner screwed through the floor and to the bottom below it.

http://webpages.charter.net/tbradshaw/Sails%20and%20Plans/drift-boat6.jpg

wizbang 13
06-19-2014, 08:46 PM
Alaska Yellow Cedar.
You have modified the construction material and the building style already.
Modify the keel , absolutely!
Lam it with AYC.
Epoxy laminated ayc is just over the top for keel, stem/stern, knees, planking, decks, masts,....

wizbang 13
06-19-2014, 08:50 PM
Oh I left out frames, dingy, oars, bowsprit, pet pillow, booms, sprits,...did I mention floors?

cerveza_fiesta
06-20-2014, 10:37 AM
@wizbang

I've heard this "make er all out of cedar" mantra before. Do you know what the typical scantling increase is when substituting cedar for hardwood structural members?

@all

To be clear, I'm talking about a cedar strip plank - not a "stripper" as such with a 1/4" thick core sandwiched in FG.

For instance, the melonseed drawings I'm looking at (John Brady's 16' version) call for 7/16 carvel or lap planking in cedar, riveted to white oak ribs, with a honduras mahogany keel, white oak transom frames, cedar transom planking and a white oak stem. Everything else structural on the boat is spec'd in white oak, except some of the visible knees are mahogany (which I assume is more for looks than anything). Centreboard is white oak. The hull is painted or varnished as desired. That is the "traditionally built" spec. I can see that anything submerged is mahogany or cedar, while the oak components are kept nominally dry.

For another instance, the VINTAGE (atkins) dinghy drawings I'm also looking at call for 3/8" cedar lap or carvel planking on w. oak frames, keel, stem solid transom, and pretty much everything else structural in white oak. Daggerboard is in white oak. Again, this is the "traditionally built" spec. On this spec, oak is used on both submerged and dry components.

====

my notion is to do a cedar core FRP build of these type of hulls. That means a slightly thinner planking (though not "stripper" thin) and everything sheathed in glass and bathed in epoxy both inside and out. The keel plank in either build is included in the epoxy sheath of course and nominally is kept dry. I want to use this construction method because A) I don't care if it's traditionally built, and B) clear cedar comes in 8' lengths max around here, usually with a lot of sapwood and I don't want to spend a fortune importing the absolutely ideal thing for a traditional build.

Maybe that's a little more background for my question. With that in mind, the question essentially was: I were to deviate from the specified traditional construction method and have a fully encapsulated FRP cedar core hull, what would be the appropriate thing to replace the hardwood structural components with?

Seems I have a possible answer in the mahoganies anyway. The honduras variety is in the original spec and so is obviously strong enough for my purposes, and I can count on the epoxy bonding to it. I get the "you lose your advantage in weight" point, but wouldn't I want the weight so that the boat rests on its design WL anyway?

wizbang 13
06-20-2014, 10:50 AM
AYC is the strongest of cedars.
it is quite a different animal than WRC or any white cedar or cypress

Yeadon
06-20-2014, 10:55 AM
I'd second AYC. If not that, then fir might be fine too.

cerveza_fiesta
06-20-2014, 11:23 AM
Any good ideas on scantling changes if one were to swap the hardwood for AYC or fir?

"Elements of Boat Strength" doesn't exactly cover hulls in this size class as far as I know.

Doug Schultz
06-20-2014, 12:42 PM
A lot of fishing boats were built here in Vancouver with AYC for planks and Fir for framing. I dont see much difference in size of frames from boats framed with oak. Maybe 10% bigger. The nice thing about the Fir is that most of the frames seem in good shape 50 or more years later where my Oak frames have been sistered twice in some areas.

wizbang 13
06-20-2014, 03:53 PM
If the ayc is laminated, just about as strong. Def stronger than rotten broken oak.

cerveza_fiesta
06-23-2014, 07:26 AM
OK, good to know. I'm not sure how much it will cost to source boat-quality AYC or doug fir at my end of the continent but I'll definitely look into it.

I have a contact in Harry Bryan (St. George NB) that does a lot of work with cedar (local and otherwise). Probably a good idea to check with him too.